Preliminary Design of the CLIC Drive-Beam Transfer Line

In the drive-beam generation complex of CLIC there is an important beam transfer line between the drive-beam accelerator and the drive-beam decelerators, where the 30 GHz RF power is generated in the decelerator structures. The design proposed for this transport system is based on building blocks or beam optics subsystems, which have been individually studied in detail and can be combined in order to cover specific functions. One function consists of bending the beams wherever required by the geometrical layout, so as to preserve the bunch length and keep the bending arc compact and compatible with acceptable synchrotron radiation. Other functions are to adjust the path length of each drive beam for synchronism with the main-linac beam and to compress or stretch the bunch according to the needs. Furthermore, there are vertical and horizontal beam translations, isochronous or acting as a compressor, and β-function transformers for matching the optics. All these functions are necessary in the drive-beam transfer that precedes injection into the decelerators. 1 MAIN FUNCTION DESCRIPTION The different parts of the beam transport system between the drive-beam accelerator and the many drive-beam decelerators (making up the CLIC RF power source [1]) must in general terms cover four types of functions: 1. Bending the beams where required in order to follow the geometrical lay-out of the drive beam generation complex, in such a way that the bunch length is preserved and the bending arc is as compact as possible, compatible with tolerable synchrotron radiation effects. 2. Adjusting the path length of each individual drive beam in order to regulate of the synchronism of the beams with the main linac beam, when they are injected into the separated decelerating sections. 3. Compressing and also stretching the bunch length according to the needs at the different stages of the beam acceleration and multiplication. 4. Vertical or horizontal beam translation, isochronous or combined in specific cases with a bunch compression. Each of the four functions have been studied and are present in various places of the drive beam generation complex. Function 1 appears each time the beam has to be bent, for instance after the pre-acceleration, between the combining rings and mainly in the “turn-around” loop preceding the injection in the decelerator. Function 2 is only required before the injection into the decelerator while Function 3 is essential into the accelerating linac and immediately after the “turn-around” loop in order to satisfy the conditions assumed for the bunch length, considering the drive beam stability. Function 4 serves mainly for the vertical translation needed after the turnaround because of the geometry adopted in the tunnel, but also for the incoming drive beam which has to be lifted up to the level of the “turn-around” loop. The whole complex which requires the four functions is briefly described below. 2 OVERVIEW OF THE TURN AROUND. The drive beam accelerator and the combiner rings are planned to be in a central position with respect to the two main linacs of the collider, which means that all the drive beams have to be first transported in a direction opposite to the main beams, before being turned around through a 360 loop and injected into the different decelerators [1] where they travel parallel to the main beams. The transport line for the beam going upstream is of course situated in the same tunnel as the decelerators, near the highest point in order to minimise the loss of space in the accessible area (Fig. 1). This position offers the advantage of keeping the “turn-around” loops near the roofs of the tunnel and of the alcoves which will house the loops. This prevents geometrical interference with the main linac and the decelerators which are placed on a common concrete support (Fig. 1), at about 1 m above the level of the tunnel floor. The difference in elevation of the beams going upstream and downstream (1.5 m approximately) imposes the need for vertical bends to bring the drive beams down before their injection in the decelerators. In addition, the up-going beam is not exactly above the down-going beam of the power-linac, since the two have to run anti-parallel over a short distance near the roof (where the path-length chicane is foreseen); they are horizontally separated by 0.75 m. In addition, the transport line carrying the up-going drive-beams must run without interruption all the way to the starting point of the main linac (also the injection-point of the first drivebeam). This transport line must therefore be placed slightly below the level of the turn-around loop (0.25 m) to avoid a crossing at the same level. Each drive-beam entering its specific loop is therefore deflected vertically. The relative vertical positions of the different beam lines are shown in Fig. 1 and in the elevation of Fig. 2. The latter shows the location of the vertical bends bringing the beam from roof-level to decelerator level just above the beam-dump line of the preceding drive-beam. 0-7803-5573-3/99/$10.00@1999 IEEE. 1638 Proceedings of the 1999 Particle Accelerator Conference, New York, 1999